Polyglycolic acid is a resin material having excellent biodegradability, gas-barrier property, strength, and the like, and has been used, in a wide range of technical fields, as medical polymer materials such as surgical sutures and artificial skins; packaging materials such as bottles and films; and a resin material for various industrial products such as injection molded articles, fibers, vapor-deposited films, and fishing lines.
Polyglycolic acid can be obtained by dehydration polycondensation of glycolic acid. However, the polyglycolic acid obtained by this method has a low degree of polymerization and a weight average molecular weight of 20 thousand or less. Although this polyglycolic acid has an excellent biodegradability, characteristics such as gas-barrier property, strength, and durability of the polyglycolic acid are not sufficiently satisfactory for many fields.
For this reason, polyglycolic acid is generally produced by ring-opening polymerization of glycolide. This method makes it possible to easily control the degree of polymerization of polyglycolic acid, and to obtain polyglycolic acid having a high degree of polymerization and a weight average molecular weight exceeding 20 thousand. The glycolide obtained here is generally synthesized as follows. Specifically, glycolic acid is subjected to dehydration polycondensation according to the following formula (I) to thereby synthesize a glycolic acid oligomer having a low degree of polymerization:
Next, the above-mentioned glycolic acid oligomer is depolymerized according to the following formula (II):

As methods for producing glycolide by such depolymerization of a glycolic acid oligomer, for example, methods for producing glycolide are proposed in which a mixture containing a glycolic acid oligomer and a high-boiling point polar organic solvent is heated to thereby dissolve the glycolic acid oligomer; the heating is continued in this state to thereby depolymerize the glycolic acid oligomer; the formed glycolide is distilled off together with the high-boiling point polar organic solvent; and then the glycolide is recovered from the distillate [for example, Japanese Unexamined Patent Application Publication No. Hei 9-328481 (PTL 1), International Publication No. WO02/014303 (PTL 2), and International Application Japanese-Phase Publication No. 2004-523596 (PTL 3)]. In particular, PTL 2 discloses that the use of a specific polyalkylene glycol ether as the high-boiling point polar organic solvent makes it possible to suppress thermal degradation of the high-boiling point polar organic solvent. In addition, PTL 2 discloses that the depolymerization reaction does not require any catalyst for the depolymerization. Moreover, PTL 3 discloses that glycolide can be efficiently produced stably for a long period by continuously or intermittently introducing the glycolic acid oligomer into a depolymerization reaction system and further by conducting the depolymerization reaction in the presence of a compound having an alcoholic hydroxyl group.
However, impurities such as glycolic acid and an open-chain dimer thereof are formed in such a depolymerization reaction of a glycolic acid oligomer, in addition to glycolide, which is the target component. Accordingly, these impurities have to be removed in order to obtain a high-purity glycolide. In addition, the impurities not only may lower the purity of glycolide, but also may cause blocking of a production line in a case where the production is continued for a long period. For this reason, it is necessary to suppress by-production of glycolic acid, an open-chain dimer thereof, and the like in the depolymerization reaction of the glycolic acid oligomer, and to thereby increase the purity of the glycolide after the depolymerization reaction.